Naturally occurring hydrocarbons often contain a portion of comparatively high molecular weight polynuclear aromatic, heteroatom (i.e. other than carbon and hydrogen, e.g. sulphur, oxygen, nitrogen and various metals) containing compounds, generally referred to collectively as asphaltenes. Such asphaltenes as contained in the hydrocarbons are harmful in remarkably decreasing the activity of hydrogenation or cracking catalysts, for example, due to the metals included in the asphaltene. Associated with the asphaltenes is a far larger fraction of similar compounds, known collectively as resins, which differ from the asphaltenes primarily in being lower molecular weight, less polynuclear aromatic, more soluble in aliphatic hydrocarbon, and far lower in harmful metal content. For this reason, it is often necessary to remove the asphaltenes, but not the resins, from asphaltene containing hydrocarbons, or to remove the resins separately.
The removal of the asphaltenes from asphaltene containing hydrocarbons is generally by a solvent deasphalting method, comprising the use of low boiling, liquid or supercritical paraffinic hydrocarbons, such as propane, butane or light naphtha, to precipitate the less soluble asphaltenes from the more soluble resins and oils, and then separate and remove them from the supernatent on the basis of their greater density, typically using gravitational settling.
The solvent may be selected from the group comprising saturated, aliphatic hydrocarbons having from 2 to 20 carbon atoms, alone or mixed; distributions of hydrocarbons from distillation cuts, with hydrocarbon chain lengths from 2 to 20 carbon atoms; and mixtures of all of the hydrocarbons cited above.
Deasphalting can be carried out in a single stage, in which case an oil phase and an asphaltic phase are obtained, the former containing both the oils and the resins. To recover the separated asphaltenes, all dissolved and dispersed alkane solvent must be removed, generally by letting down the pressure to convert the liquid or supercritical alkane to a gaseous or subcritical state. Entrained alkane droplets left in the asphaltenes by incomplete separation causes increased, often uncontrollable foaming during solvent removal. This causes contamination of the solvent and subsequent fouling of the equipment.
Deasphalting can also be carried out in two stages, using different solvents and/or different operating conditions, such as temperature, pressure or dilution, in each of the two stages. In the two-stage process, the oils and the resins in the single stage supernatent are separated from each other in a second stage. Thus any precipitated but unseparated asphaltenes in the supernatent going to the second stage contaminates and degrades the quality of the recovered resins.
The recovered asphaltenes are used to make a hard pitch that can be ground and used as a solid fuel, or to make an asphalt binder that can be mixed with aggregate and used to pave roads. Resins and oils can be carried into the asphaltenes with the entained solvent and left behind when the solvent is removed. Contamination of the pitch or asphalt with even minor amounts of the less viscous, more creepy resins and oils degrade the quality of the pitch or asphalt.